The invention relates to an electrical insertion body for a tube lamp which seals a sealing tube of a tube lamp, such as a mercury lamp, a metal halide lamp, a halogen lamp or the like. The invention furthermore relates to a production process for the electrical insertion body. The expression xe2x80x9celectrical insertion body for a tube lampxe2x80x9d is defined as an arrangement in which a sealing body is combined with an upholding part of the electrode.
In a tube lamp, for example a high pressure discharge lamp, in a spherical or oval fused silica glass arc tube there are a pair of electrodes opposite one another and the tube is filled with an emission metal such as mercury or the like, discharge gas and the like. Cylindrical sealing tubes are connected to the ends of the arc tube. Upholding parts of the electrodes with tips each provided with an electrode, and outer lead pins are electrically connected by these sealing tubes and are sealed in this state. Since however the upholding parts of the electrodes of tungsten and the sealing tubes of fused silica glass have very different coefficients of thermal expansion, the sealing tubes cannot be directly welded to the upholding parts of the electrodes and sealed.
The sealing tubes were therefore conventionally sealed by a foil sealing process, a step joining process or the like. In the step joining process several types of glass with different coefficients of thermal expansion are joined to one another. Recently it has become more and more important to seal sealing tubes which are connected to the ends of the arc tubes using sealing bodies which consist of a functional gradient material which consists of a dielectric inorganic material component such as silicon dioxide or the like and of an electrically conductive inorganic material component such as molybdenum or the like and which is made essentially columnar.
In this sealing body one end is rich in the dielectric inorganic material component such as silicon dioxide or the like and in the direction to the other end the proportion of electrically conductive inorganic material component such as molybdenum or the like increases continuously or in steps.
In a sealing body of a functional gradient material which is formed from silicon dioxide and molybdenum, therefore the vicinity of one end of the sealing body contains a large amount of silicon dioxide, is dielectric and has a coefficient of thermal expansion which is roughly equal to that of the fused silica glass, while the vicinity of the other end contains a large amount of molybdenum, is electrically conductive and has the property that its coefficient of thermal expansion approaches that of the molybdenum.
Since in this sealing body of a functional gradient material the gradient of the change of the ratio of the dielectric inorganic material component to the electrically conductive inorganic material component can be increased, the one face side has a large proportion of the dielectric inorganic material component while the other face side can have a large proportion of the electrically conductive inorganic material component, even if the sealing body is not long in its axial direction.
The functional gradient material has no interface on which the composition of its material components changes significantly. The functional gradient material is therefore resistant to thermal shock and has high mechanical strength. Therefore the locations to be sealed at which the sealing tubes and the sealing bodies are welded to one another approach the center area of the arc tube which reaches a high temperature during operation. Therefore there is the advantage that the length of the sealing tubes can be decreased, the short length of the sealing tubes in the axial direction also contributing to this advantage.
If the sealing body is formed from a functional gradient material of the electrically conductive inorganic material component and the dielectric inorganic material component, the following is done.
First a binder is added to these powders. By pressing it in a casting mold a columnar compact is obtained which is temporarily sintered at a temperature of roughly 1300xc2x0 C. In this way a temporarily sintered body is obtained.
Next, drilling is done to produce a center opening which is used to insert the upholding part of the electrode into the center axis of this temporarily sintered body.
Alternatively, pressing is done in a casting mold with a projecting component for forming the center opening. Thus a compact with a center opening produced beforehand is obtained. It is temporarily sintered. The upholding part of the electrode is inserted into the center opening of the temporarily sintered body. Afterwards complete sintering is done at a temperature of roughly 1750xc2x0 C.
Since these materials shrink during sintering of the functional gradient material by 10 to 20%, it is necessary for the center opening of the temporarily sintered body to be made larger than the outside diameter of the upholding part of the electrode. If here the size of the center opening is not enough, during complete sintering in the functional gradient material a stress forms around the upholding part of the electrode, as does subsequent cracking. Therefore the center opening must be made somewhat larger than a stipulated value and in this way cracking is prevented even if the functional gradient material shrinks due to complete sintering.
In this case the disadvantage was the following:
Due to variations of the diameter of the center opening, variations of contraction during complete sintering and for similar reasons the upholding parts of the electrodes are not arranged stably enough on the sealing bodies to tightly adjoin one another. In the case of a through opening in which this center opening penetrates one face side of the sealing body as far as its other face side, the hermetic adhesion property is inadequate. Therefore, after complete sintering on the side of the sealing body from which the upholding part of the electrode projects glass or brazing filler metal is applied as a deposit and thus leaking is prevented, this side projecting from the tube lamp to the outside. Furthermore, in this way the attachment of the upholding parts of the electrode in the sealing bodies was ensured. In this process however there was the disadvantage that the working steps increased and production required high expense.
Furthermore, in the case of a center opening which extends from one face side of the sealing body by a stipulated distance and which is therefore not made continuous, there was no problem of leakage, but there was the disadvantage that as a result of inadequate attachment the upholding parts of the electrodes fall out due to vibration or the like, or similar defects. In this case it was also necessary to take some measures to ensure attachment of the sealing bodies to the upholding parts of the electrodes after complete sintering.
Therefore the object of the invention to devise an electrical insertion body for a tube lamp in which an upholding part of the electrode is securely attached by sintering into the center opening of a sealing body of an electrically conductive inorganic material component and a dielectric inorganic material component and in which neither leaks nor falling out of the upholding parts of the electrodes occur. Furthermore the object of the invention is to devise a production process for this.
The object is achieved in the invention in an electrical insertion body for a tube lamp for hermetic sealing of the sealing tubes which are connected to the arc tube of the tube lamp in
that there are sealing bodies for the tube lamp in which one upholding part of the electrode at a time is inserted into the center opening which is provided in the sintered functional gradient material which consists of an electrically conductive inorganic material component and of a dielectric inorganic material component and which is shaped in the form of a multilayer column such that the ratio of the two components changes gradually in the axial direction,
that in the boundary areas between the above described sealing bodies and the upholding parts of the electrodes one diffusion area at a time is formed, in which the electrically conductive inorganic material component of the sealing body, the metallic component of the upholding part of the electrode and a diffusion accelerator are present diffused into one another, which at the sintering temperature of the above described functional gradient material accelerates diffusion of the electrically conductive inorganic material component of the sealing body and of the metallic component of the upholding part of the electrode and
that in this way the upholding part of the electrode and the inside of the center opening of the sealing body are joined to one another.
The term xe2x80x9cdiffusion acceleratorxe2x80x9d in the invention is defined as a material which, at the sintering temperature of the functional gradient material which forms the sealing body, dissolves in the metallic component of the upholding part of the electrode and also in the electrically conductive inorganic material component of the sealing body and accelerates diffusion of the above described electrically conductive inorganic material component and the electrically conductive inorganic material component of the sealing body into one another.
One such electrical insertion body for a tube lamp is advantageously produced by the process of the present invention.